svn commit: r324313 - head/sys/amd64/amd64

[Konstantin Belousov]

On Sat, Oct 07, 2017 at 01:54:05PM -0700, Don Lewis wrote:
> On  7 Oct, Konstantin Belousov wrote:
> > On Sat, Oct 07, 2017 at 01:04:09PM -0700, Don Lewis wrote:
> >> On  5 Oct, Konstantin Belousov wrote:
> >> > Author: kib
> >> > Date: Thu Oct  5 12:50:03 2017
> >> > New Revision: 324313
> >> > URL: https://svnweb.freebsd.org/changeset/base/324313
> >> > 
> >> > Log:
> >> >   Avoid a race betweem freeing LDT and context switches.
> >> >   
> >> >   cpu_switch.S uses curproc->p_md.md_ldt value as the flag indicating
> >> >   presence of the process LDT.  The flag is checked and then ldt segment
> >> >   descriptor is copied into the CPU' GDT slot.
> >> >   
> >> >   Disallow context switches around clearing of the curproc LDT state by
> >> >   performing the cleanup in critical section.  Ensure that the md_ldt
> >> >   flag is cleared before md_ldt_sd descriptor content is destroyed by
> >> >   inserting fence between the operations.
> >> >   
> >> >   We depend on the x86 memory model strong ordering guarantees, in
> >> >   particular, that cpu_switch.S observes the writes to md_ldt and
> >> >   md_ldt_sd in the expected order.
> >> 
> >> I don't know which of this series of commits is responsible, but I think
> >> that it fixed the build of lang/ghc on Ryzen.
> >> 
> >> https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=221029#c102
> > 
> > Does ghc use LDT on amd64 ?  This sounds unbelievable.
> 
> I have no idea, but ghc would reliably fail to build on my Ryzen machine
> up through r323398, and at r324367 it seems to reliably build.
Could you try to bisect ?

I reviewed all kernel changes and do not see anything which could be
marked as possible fix for whatever kernel issue causing usermode
fault.